1. Field of the Invention
This invention relates to an ionically conductive material and to a method for its preparation.
2. Background Information
The development of electronic devices such as batteries, cells, display elements (ECD, etc.), and so forth has continued over the last several years in terms of higher performance, smaller size, and smaller thicknesses. In association with these developments, it is of course necessary that the ionically conductive materials used therein exhibit high performance, but they must also satisfy various advanced requirements such as solidification, high reliability, high flexibility, high moldability, moisture resistance, and so forth.
Ionically conductive materials already known in this regard are, inter alia, as follows:
(i) electrolyte solutions comprising electrolyte dissolved in water, aqueous solvent, or organic solvent; PA1 (ii) solid electrolytes composed of an inorganic such as beta-alumina (beta-Al.sub.2 O.sub.3), lithium nitride (Li.sub.3 N), lithium iodide/alumina (LiI-Al.sub.2 O.sub.3), rubidium silver iodide, and so forth; PA1 (iii) solid electrolytes in which the salt of a metal from Group I or Group II of the Periodic Table is dissolved or dispersed in a macromolecular or polymeric resin matrix. PA1 (a) the quantity of dissolution of contained electrolyte (metal salt) must be suitably large, and the capacity for ionic dissociation must be large; and PA1 (b) the dissociated ion must be highly mobile in the polymer matrix. PA1 component (D) is added to the mixture of component (A) and component (B) followed by heating to afford the crosslinked copolymer constituted of components (A) and (B), this is then swelled with organic solvent, and component (C) is admixed and dispersed; PA1 the mixture of components (A) through (D) is heated in order to produce the crosslinked copolymer of components (A) and (B) while at the same time achieving the dispersion of component (C).
However, due to the use of a liquid such as water or organic solvent in the material, electrolyte solutions (i) inevitably suffer from the problem of liquid leakage to the exterior of the electronic device. This leakage may cause the performance of the device to deteriorate and may damage the surroundings. In order to ameliorate this problem, ionically conductive materials in paste or gel form have been prepared by mixing polymeric or macromolecular compounds into electrolyte solutions. Nevertheless, even these materials do not completely eliminate the leakage problem. The solid electrolytes under (ii) are in fact technically applicable to highly reliable, long-lived electronic devices, and, moreover, respond to the demands for small size and increasing thinness. However, materials with a satisfactory room-temperature conductivity cannot be obtained at present, with the result that broad practical application remains unrealized. With regard to the solid electrolytes under (iii), while these can essentially solve the leakage problem as for (ii), they also offer the prospect of imparting the excellent properties characteristic of organic polymers such as high flexibility and high moldability. They have therefore received attention as materials which can respond to the broad demands of electronic devices as described above. In this regard, polymeric ionically conductive materials for use as solid electrolytes must meet the following demands:
Due to their relatively good conductivity, crosslinked materials containing a polyether segment (for example, PEO= polyethylene oxide, and so forth) have been extensively examined as polymer structures which satisfy the preceding conditions. However, when PEO is itself simply crosslinked, the molecular mobility is limited, and a satisfactory room-temperature conductivity is not obtained. Accordingly, in order to solve this problem, experiments have been carried out into the synthesis of solid electrolytes which combine the PEO segment with a siloxane segment (segment with very high molecular mobility). For example, Japanese Patent Application Laid Open [Kokai] Number 60-216463 [216,463/85], Japanese Patent Application Laid Open Number 60-217263 [217,263/85], and U.S. Pat. No. 4,798,773, issues Jan. 17, 1989, describe ionically conductive materials in which the lithium ion, etc., is dispersed in a crosslinked copolymer of PEO and siloxane in which linkage is achieved through the Si--O--C bond. However, the handling of these ionically conductive materials is made highly problematic by the facile scission of the Si--O--C bond in the presence of water. Otherwise, an ionically conductive material has been described in which metal ion is dispersed in a polysiloxane with polyethylene glycol side chains which has been crosslinked and solidified with difunctional isocyanate (Solid State Ionics, 15 233 (1985)). In this case, the NCO group must be added in a suitable excess relative to the OH group in order to induce curing to a satisfactory strength level. Thus, when utilized in a device such as a battery, etc., the risk arises of a reaction between residual NCO groups and the electrode material, and this problem impairs practical utilization. Furthermore, Japanese Patent Application Laid Open Number 62-209169 [209,169/87]lists crosslinking by exposure to radiation (electron beam, etc.) and crosslinking by the platinum catalyst-mediated hydrosilylation reaction as methods for the preparation of siloxane/PEO crosslinked materials, and also discloses ionically conductive material in which metal ion is dispersed in same. However, these methods require the use of organic solvent in order to compatibilize the two or more types of starting materials, which not only closes off application in some products, but also increases the complexity of the process. Among other things, these methods also risk a deterioration in the occupational environment, damage to surrounding materials, and organic solvent residues in the final product. Moreover, even given that the starting materials are miscibilized by organic solvent, phase separation can occur during the course of organic solvent evaporation, and the complete development of the crosslinking reaction can therefore not be thoroughly guaranteed. Considered in total, these methods suffer from problems in their practical application due to a reduced product quality, unsatisfactory reproducibility, and so forth.
As discussed above, the heretofore proposed solid electrolytes composed of crosslinked material from organopolysiloxane and another polymer in all cases suffer from problems with their properties or preparative method and are therefore unsuitable for application in electronic devices as listed above.
The present inventor, as the result of extensive research directed at solving these problems, discovered that material comprising a specific metal ion dispersed in a specific crosslinked copolymer does not suffer from the problems outlined above and has an excellent ionic conductivity. The present invention was achieved based on this finding.